Most plotters of the prior art have some form of X and Y tracking mechanism which transports a carriage or marker along orthogonal axes to a desired point on a plotting surface. The tracking mechanism is usually a rigid structure which encloses the plotting surface, thus making the maximum area of the plotting surface dependent upon the size of the tracking mechanism. One such plotter, for example, is disclosed in Neasham U.S. Pat. No. 3,355,730 issued Nov. 28, 1967 in which the tracking mechanism consists of a pair of elongated precision lead screws mounted at right angles to each other along the length and width of a plotting surface. The lead screws are rotatably driven by respective motors, and each have a parallel guide rod mounted along the opposite boundary of the plotting surface. A pair of perpendicular wires, representing the X and Y coordinates are each suspended between a lead screw and a guide rod. By rotating the lead screws, the positions of the wires are changed to correspond to respective X and Y data. The intersection of the two wires mark the desired point.
The plotters embodying a major disadvantage stemming from the use of the rigid X and Y tracking mechanisms, in that any increase in the area of the plotting surface must be accompanied by an increase in the overall dimensions, precision, weight, and size of the tracking mechanism. In Neasham, for example, any increase in the area of the plotting surface requires the use of longer and heavier lead screws and guide arms, which in turn require larger and heavier motors consuming more power. As a result, the price of these devices bears a direct linear relationship to the area of the plotting surface.
Alternate types of plotters have been suggested. These plotters are based upon triangular coordinates in which an unknown point is represented by the intersection of two radii originating from two known separated fixed points. One such plotting device is disclosed in the Huckabay et al U.S. Pat. No. 3,184,740 issued May 18, 1965 in which a pair of telescoping arms are each pivotally connected at respective ends to fixed separated points. The other respective ends are pivotally interconnected. A light is mounted at the point of interconnection to illuminate the desired point on the plotting surface. A similar device is disclosed in Birrell U.S. Pat. No. 3,230,622 issued Jan. 25, 1966, except that a pair of elongated lead screws is used instead of telescoping arms. As these triangular coordinate plotters also utilize a rigid tracking mechanism, they suffer from the same disadvantages as the X and Y plotters.
Linn U.S. Pat. No. 3,564,533 issued Feb. 16, 1971 for a triangular graphic digitizer, shows a device used to obtain the relative positions of points on a graphic image. In the arrangement disclosed, one of the ends of each of two cables are pivotally connected to a common handpiece. Each cable then passes over a corresponding pulley, mounted to swivel on an axis perpendicular to the plane containing the graphic image, and is wound around a respective encoder to which the opposite ends are affixed. As an operator moves the handpiece over the points in a graphic image, the changes in the lengths of the cables are recognized by the encoders and recorded. Linn performs a function opposite to that of a plotter. A plotter uses information to create a graphic image while Linn obtains information from a graphic image. No means are disclosed in Linn for using the digitizer as a plotting device. In fact, the arrangement disclosed in Linn could not operate as a plotter, in that the handpiece (considered as a marking device) will not retain its orientation unless held in place by the operator. No means are supplied for supporting the handpiece as it moves over the graphic image.